High-Performance Liquid Chromatographic Enantioseparation of Atropisomeric Biphenyls on Seven Chiral Stationary Phases

The HPLC enantioseparation of eight racemic atropisomeric biphenyls on commercially available polymeric Chiralcel OD-H, Lux Cellulose-1, Lux Cellulose-2, Chiralcel OJ, Lux Amylose-2 and Chiralpak OT(+) and on the brush-type Whelk-O1 columns, both in normal-phase mode and in polar organic solvent mode, has been investigated. The attempts to enantioseparate the selected biphenyls on Whelk-O1 were unsuccessful. All compounds were well resolved on almost one of the polymeric columns. Lux Cellulose-2 showed to be suitable for enantioseparation of all biphenyls. The effect of mobile phase, temperature, type of chiral selector and analyte structure on enantioseparation are examined and discussed. 2-Propanol and ethanol were employed as mobile phase modifiers and their influence on the retention and enantioseparation was investigated. Also a ternary mobile phase (n-hexane/2-propanol/methanol 91:6:3) was employed to test the separation of the eight biphenyls. In same cases, the elution with pure ethanol provided good enantioseparation in shorter elution times. The experimental data evidenced the complemental chiral recognition capabilities of polysaccharide-based CSPs. Noteworthy, Lux Cellulose-1 and Chiralcel OD-H contain the same chiral selector, but the first one exhibited higher retention factors. The evaluation of chromatographic data provided information about the chiral recognition mechanisms. In this regard, we report on the effect of ortho and meta biphenyl substituents on the retention and enantioseparation. In addition, computational evaluation of electrostatic potentials of analytes furnished a very interesting piece of information about the enantioseparability as well as the chiral recognition mechanisms on the evaluated chiral selectors.